1. Field of the Invention
The present invention relates to a digital transmission apparatus which is able to relay different types of trunk signals, for example, a digital transmission apparatus which directly relays exchange trunk signals and pulse "ear and mouth" (E&M) signals.
The trunks provided at the input and output ports of exchanges are essential components in the interfaces for connecting one exchange with another. A digital transmission apparatus is inserted between the trunks of one exchange and the trunks of the other exchange for actual connection of the same. One such transmission apparatus is the one known as a "PCM30", which for example functions to multiplex, demultiplex, and transmit 30 channels' worth of signals. The present invention relates to a functional facility used in such a digital transmission apparatus, that is, a transmission apparatus for relaying different types of trunk signals.
2. Description of the Related Art
As will be explained in further detail later referring to the drawings, a general telephone network includes a plurality of so-called "subscriber side exchanges", i.e., lower rank exchanges each having a plurality of subscriber terminals under them connected by subscriber circuits.
These lower rank exchanges connect to subscriber terminals of other lower rank exchanges through higher rank exchanges called "regional centers (RC)". Such higher rank exchanges RC are constructed at respective regions. When establishing an intra-city route connecting one higher rank exchange RC with another over a steep mountain or dense jungle, the two higher rank exchanges RC are connected to each other through radio stations.
To enable transfer of trunk signals between one lower rank exchange and another through a pair of radio stations in such a network configuration, it is necessary to use two different types of signals, that is, the digital trunk signals of the exchange trunk signals (S.sub.xt) and the analog trunk signals of the pulse E&M signals (Se.sub.m).
The transfer of information among these two completely different types of trunk signals, i.e., the exchange trunk signals S.sub.xt and pulse E&M signals Se.sub.m, is possible due to the higher rank exchanges RC, which serve as interfaces between these signals. Without such higher rank exchanges RC, relaying between these different types of trunk signals has not been possible. Therefore, there was the problem that large sized facilities were required, which pushed up line costs.